Questions about extension tubes

[Bob Blaylock]

I'm not sure if you've purchased them yet or not, either, but be aware that even with the Kenko DG (?) tubes that have the contacts and such, you MAY lose the autofocus. The camera will try, but if it just sits there and jitters back and forth with the focus a small amount, you'll have to manually focus. If you have or buy the Kenko tubes, and they do this, I recommend calling Kenko USA (presuming you're in the USA), and talking to technical support... It may not help, but you never know.

Using my D3200, my 18-55mm “kit lens”, and my generic set of extension tubes, (12mm, 20mm, 36mm), I have found that I can still get autofocus with any one of these tubes, and with any combination of two of them except the 20mm and the 36mm). So, up to 48mm of extension, I get autofocus, at 56mm or more, I don't.

I guess I should count myself lucky I get any autofocus at all. The manual for my D3200 specifies that autofocus requires a largest effective aperture of at least ƒ/5.6. The extension tubes are only really useful with the lens zoomed in to 55mm, at which ƒ/5.6 is the largest effective aperture of the lens by itself, and surely adding any extension tubes must have the effect of reducing the effective aperture somewhat; so when I am using my extension tubes, and getting autofocus, then at that point, I have my camera performing beyond the limits that Nikon claims for it.

 

[Bob Blaylock]

Highly unlikely..........let me rephrase that........NO

Your extension tubes essentially increase the focal length of the lens you are using…

Um, no it doesn't. If it is a 100mm lens without the tubes, it's still 100mm with them. Same thing when using bellows, or focusing any camera that uses bellows (folders, medium and large format cameras etc.)

…by moving the lens' focal point farther forward…

Again, technically an incorrect statement. Well, maybe not 'incorrect', but certainly not based on optical physics.

Extension tubes and bellows don't do anything to a lens' focal points. Any lens has an infinite number of possible focus points at both paths along the optical line in 'front' and 'behind' the glass elements. What extension tubes do is allow one to choose a focus plane in 'front' of the lens that is closer to the camera by physically moving the lens further from the focus plane where the film of sensor sets. But the lens still has the same infinite number of focus points. Most are registered in the image as out-of-focus. Yet the focal length is still unchanged.

A 100mm lens, focused at infinity, has its optical center 100mm away from the sensor, and has an angle of view that is based on the arccosine of the size of the sensor divided by twice that focal distance.

The simplest lenses focus by moving the entire lens away from the sensor, to focus on closer subjects. As the lens moves farther from the sensor, the angle of view becomes narrower. Move a 100mm lens out to 200mm away from the sensor, and now you're focused on something very close, and you have a narrower angle of view—the same angle of view than a 200mm lens would have when focused at infinity.

This is the phenomenon known as “focus breathing” or “lens breathing”. More sophisticated lenses try to minimize it by moving different elements in different ways, rather than by moving the whole lens at once. But when you use extension tubes, you're moving the whole lens at once. Stick 100mm of extension tubes on a 100mm lens, and no matter what that lens does to avoid breathing due to its own focus, you're still getting the full extra 100mm of focus breathing, and getting the angle of view of a 200mm lens. This isn't really a terrible thing, when you consider that you'd usually be using extension tubes to try to take detailed pictures of very small objects, and in that context, the narrower angle of view is helpful.

 

[480sparky]

A 100mm lens, focused at infinity, has its optical center 100mm away from the sensor, and has an angle of view that is based on the arccosine of the size of the sensor divided by twice that focal distance.

The simplest lenses focus by moving the entire lens away from the sensor, to focus on closer subjects. As the lens moves farther from the sensor, the angle of view becomes narrower. Move a 100mm lens out to 200mm away from the sensor, and now you're focused on something very close, and you have a narrower angle of view—the same angle of view than a 200mm lens would have when focused at infinity.

This is the phenomenon known as “focus breathing” or “lens breathing”. More sophisticated lenses try to minimize it by moving different elements in different ways, rather than by moving the whole lens at once. But when you use extension tubes, you're moving the whole lens at once. Stick 100mm of extension tubes on a 100mm lens, and no matter what that lens does to avoid breathing due to its own focus, you're still getting the full extra 100mm of focus breathing, and getting the angle of view of a 200mm lens. This isn't really a terrible thing, when you consider that you'd usually be using extension tubes to try to take detailed pictures of very small objects, and in that context, the narrower angle of view is helpful.

Yet a billion millimeters of extension tubes won't change the focal length of the lens.

Take the 100mm lens off a camera, and it's still a one hundred millimeter lens.